1. Field of the Invention
The present invention relates to a trench MOSFET including a gate electrode having a trench gate structure, and particularly to the trench MOSFET in which parallel schottky diodes are formed and housed between a source and a drain thereof.
2. Description of the Related Art
In a step down DC-DC converter, a MOSFET is used as a switching (chopping) element. During a period of current interruption from a primary side to a secondary side by the switching, a structure to flow back the current by a flywheel diode is generally used so as not to interrupt the current at a load side. However, in accordance with a requirement for a DC-DC converter having a low output voltage at the load side, a forward voltage drop of the above-stated diode cannot be ignored as a loss. Consequently, a structure is also used, in which a passage between a source and a drain of another MOSFET (second MOSFET) is used instead of the diode, and it is turned on for the same period as the period when the diode is conducting. As a typical MOSFET used for such a purpose, there is so-called a trench MOSFET having a trench gate structure.
In the above-stated structure, it is difficult to control a gate voltage so as to turn on the second MOSFET for the completely same period as the period when the diode is conducting, and actually, it is used to have a period when both the MOSFET for chopping and the second MOSFET are turned off (dead time). During this dead time, the second MOSFET functions as a diode (pn junction diode) which is included as a parasitic element. The period is short, but the forward voltage drop thereof becomes a problem as the loss, all the same. Consequently, to reduce the forward voltage drop during the dead time, a structure in which schottky diodes are connected to the second MOSFET in parallel, is used.
Such a schottky diode is conceivable to be connected as a different component from the second MOSFET, but it is also conceivable to house in the second MOSFET from points of view of advantages in a converter configuration. There are some points to be considered when it is housed. One of them is a cost. In this means, the schottky diode having a structure which does not occupy too many areas on a semiconductor substrate, and not making processes excessively complicated, is desirable. Besides, it is also required to consider not to disturb functions as a basic schottky diode (for example, a low resistivity, a low leakage current, and so on) or functions as a transistor by removing these factors giving influences to the cost. Incidentally, as semiconductor elements in which a schottky barrier is intentionally formed within the semiconductor, for example, there are ones described in the following respective Patent Documents.
[Patent Document 1] Japanese Patent Laid-open Application No. Hei 11-251573
[Patent Document 2] Japanese Patent Laid-open Application No. 2003-17701
[Patent Document 3] U.S. Publication No. US-A1-2003/0207538